(Reuters) – Patients with advanced breast cancer tied to an inherited gene mutation who were treated with an experimental Pfizer Inc drug went about three months longer before their disease worsened than those who received chemotherapy in a late stage study, according to data released on Friday.

The drug, talazoparib, a once daily pill that Pfizer acquired with its $14 billion purchase of Medivation, belongs to a class of medicines called PARP inhibitors that may induce tumor cell death. They have shown promise in advanced ovarian and breast cancers.

Patients in the Phase III study had mutations of the BRCA1/2 genes, the type of mutation that led actress Angelina Jolie to have preventive breast removal surgery.

About 3 percent of breast cancers occur in people with inherited BRCA1 or BRCA2 mutations that lower a cell’s ability to repair damaged DNA. Up to 65 percent of women who inherit the mutations will develop breast cancer, often much younger than is typical for the disease.

In the 431-patient trial, those who received talazoparib went 8.6 months before half of them experienced disease progression, a measure known as median progression-free survival (PFS). Among those who received standard chemotherapy, the median PFS was 5.6 months.

In addition, 62.6 percent of talazoparib patients experienced a complete or partial response to the treatment compared with a 27.2 percent response rate for chemotherapy.

Twelve patients who received the Pfizer drug, or 5.5 percent, had a complete response, meaning no detectable sign of cancer. There were no complete responses in the chemotherapy group. The results, unveiled at the San Antonio Breast Cancer Symposium, were highly statistically significant.

Researchers also reported a significant delay in time to meaningful deterioration of quality of life among talazoparib patients.

Dr. Jennifer Litton, the study’s lead investigator from MD Anderson Cancer Center, said there are currently no drugs specifically approved for this group of patients aside from standard chemotherapies.

The results were consistent whether patient had received up to three courses of chemotherapy or none at all, or whether patients’ cancers had spread to the brain.

The incidence of serious adverse side effects was similar in both groups — 31.8 percent for the Pfizer drug and 29.4 percent for chemotherapy. Discontinuations due to adverse events occurred in 7.7 percent of talazoparib patients and 9.5 percent in the chemotherapy group.